Given two optimal policies, is an affine combination of them also optimal?
Yes, in general any linear combination of probability distributions between optimal policies is also an optimal policy. In fact any combination with each state treated separately will also be an optimal policy.
This can be seen using the equation for optimal deterministic policy in terms of optimal value function:
$$\pi^*(s) = \text{argmax}_a [\sum_{r,s'}p(r,s'|s,a)(r + \gamma v^*(s'))] = \text{argmax}_a [q^*(s,a)]$$
The only way to have multiple equivalent optimal policies is when there are two or more actions tied for $\text{max}_a [q^*(s,a)]$. If that is the case, then it does not matter - in terms of expected future reward - which of those actions is taken in the affected state. It is possible to take one or the other action.
You can use any rule you wish to decide which of the tied-for-max actions to take, including a random choice. Creating a new policy that is a linear combination of optimal policies is one way to make that random choice. You could equally say that you would make one action choice on a Monday and another on Tuesday - in terms of being optimal choices it does not matter at all how you break ties.